The present invention relates to a measurement head for use with a welding robot for measuring the shape of a portion of a workpiece to be welded and the position of the workpiece portion with respect to the welding robot.
It has heretofore been conventional to use microcomputer-controlled electric spot welding robots for automatically welding relatively thin iron sheets such as in assembling automobiles. In building construction and shipbuilding, however, arc welding must be employed since considerably thicker sheets of iron, 10 mm or more thick, for example, are required to be welded. Therefore, the spot welding robots cannot be used in building and ship construction. One prior practice for such welding applications has been to use a measurement head coupled to a welding torch for detecting the position of the welding torch relative to a workpiece and moving the welding torch properly to the desired position. It has been proposed to employ an electromagnetic sensor such as a coil as the measurement head. With this prior art measurement head, it is possible to determine only the distance of the head relative to planes of iron sheets to be welded. Furthermore, to avoid thermal influences, the sensor needs to be spaced a moderate distance from the welding arc and hence cannot necessarily provide information representing the shape of a portion to be welded and an adjacent area. Accordingly, the conventional measurement head fails to effect accurate positional control of the welding torch.
Another method practiced in the art has been to weave the welding torch laterally through a given width across a workpiece portion to be welded and to adjust the position of the welding torch so that an arc current flowing at the time will vary in a symmetrical pattern. This method is effective only when metal sheets to be welded are abutted against each other to form a symmetrical configuration with respect to the joint line.
According to still another method, a fan beam of light ected on a workpiece to be welded, the resulting bright stripe on the workpiece corresponding to a sectional line thereof is picked up by an industrial television camera, the obtained image data is processed to determine the sectional line, and the welding torch is positionally controlled on the basis of shape of the sectional line. However, when observing an area closely adjacent to the welding arc, it is difficult to separate an image signal corresponding to the bright-stripe from a strong flare of the light from the welding torch.
The three prior techniques as described above are described in "Development and Application of Sensors and Sensor System for Arc Welding in Japan" (IIW XII-C-031-82) reported by I. MASUMOTO in International Institute of Welding (IIW), 1982.